130 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



selected stars. For the moon makes a rapid circuit of the sky once 

 each month and in so doing passes close to a number of bright stars. 

 Hence if the navigator can be provided with tables giving the 

 Greenwich time when the moon should be found to be distant from 

 a given star by a given amount which he has measured, then the 

 Greenwich time of the instant of that observation becomes known 

 and may be kept by an hour glass or watch for a few minutes while 

 the ship's time is being found. 



This method for determining longitude was foreseen as early as 

 1514, but its practical application was attended with difficulties, not 

 surmounted indeed before another and better method had been de- 

 veloped through the invention of the accurate chronometer. The 

 difficulties in the way of the lunar distance determination of longi- 

 tude were imperfect knowledge of the moon's motion and the crude 

 character of the instruments for measuring angles, together with 

 some inferiority inherent in the method. 



The study of the longitude problem was stimulated by a prize of 

 1,000 crowns offered by Philip III of Spain, followed by another of 

 10,000 florins by the states-general for the discovery of a method of 

 finding longitude at sea. As a result it was brought out that methods 

 depending on the moon's position offered the best solution at that 

 time, but that the lunar tables extant were useless, and that much 

 study and observation would be necessary to make them available. 

 Primarily to attack this lunar problem England established her 

 national observatory in 1675. Fifty-six years later the astronomer 

 royal in charge of the Greenwich Observatory announced that he 

 hoped to be able to compute the moon's position within such limits 

 that longitude errors would be reduced to 60 geographical miles 

 at the equator. Apparent^ progress had been slow. 



In 1714 England's commission for the discovery of longitude at 

 sea had been constituted with power to grant large sums in prizes. 

 For a method of determining the longitude within 60 geographical 

 miles, to be tested by a voyage to the West Indies and back, £10,000 

 was offered ; within 40 miles, £15,000 : within 30 miles, £20,000. 



The importance of further progress in methods of navigation at 

 this time is brought out by accounts of actual casualties showing 

 what the dangers were. Admiral Wheeler's squadron, in 1694, leav- 

 ing the Mediterranean, ran on Gilbraltar when it was thought the 

 strait was safely passed. Sir Cloudesley Shovel's squadron, in 1707. 

 was lost on the rocks off Scilly, by erring in the latitude. Several 

 transports in 1711 were lost near the St. Lawrence River, having 

 erred 45 miles in their reckoning. Lord Belhaven was lost on the 

 Lizard in 1721, the same day on which he sailed from Plymouth, 

 England. 



